| Purple loosestrife, Lythrum salicaria, is a Eurasian plant that has invaded North American wetlands over the last 200 years. As a precautionary measure, but without detailed knowledge of the ecological impact of Lythrum on native species, a biological control project was started in the 1990s with the introduction of Galerucella calmariensis, a leaf-feeding beetle of European origin.; To measure the ecological impact of Lythrum, I assessed the effect of Lythrum on a rare, native plant species, Sidalcea hendersonii. To evaluate the biological success of the biological control project, I investigated the effectiveness of the biological control agent and identified factors associated with among-site variability in defoliation levels.; Lythrum's impact on Sidalcea was not stronger than the impact of native species, possibly owing to the early emergence in the spring and the tall growth of Sidalcea. The abundance of Lythrum in a marsh where it co-occurs with Sidalcea did not increase significantly from 1979 to 1999, which suggests that the invasion process was largely completed by 1979.; The effectiveness of the control agent was dependent on site condition. In tidal areas, the beetles could not establish due to the tidal disturbances. At sites where the control agent established, the reproductive performance and dry biomass of Lythrum were strongly reduced by beetle feeding damage in the same year, but stem density only declined at one site, Chilliwack, after severe herbivory over a four-year period. At most other non-tidal sites, plant stem density increased, and stem density changes were inversely related to the average beetle feeding damage during the previous four years.; Beetle feeding damage was associated with predation on G. calmariensis eggs, but not predation on any other life-history stage, or plant quality. The densities of crawling, invertebrate predators were not generally related to predation levels, suggesting that predation did not vary in response to predator abundance, but to predator attack levels.; I discuss the level of success of this biological control project and the need for control methods in tidal areas. |
